Rubber compositions can be reinforced to show enhanced modulus (complex elastic modulus) by incorporating microfibrillated plant fibers such as cellulose fibers as filler into the rubber compositions. However, since microfibrillated plant fibers are highly self-aggregative and poorly compatible with rubber components, they show low dispersibility during rubber kneading. For this reason, the incorporation of microfibrillated plant fibers may deteriorate fuel economy or other properties. There is also a need for methods for improving the dispersibility of microfibrillated plant fibers.
Patent Literatures 1 to 4 disclose that rubber properties and other properties are improved by previously mixing cellulose fibers and a rubber latex to improve the dispersibility of the cellulose fibers. However, there is still room for improvement to further enhance the dispersibility of cellulose fibers in a masterbatch containing rubber and the cellulose fibers in order to improve rubber properties and other properties.